Image noise is an important parameter in the quality of medical diagnosis. Several scientific studies have indicated that even slight increase of noise in medical images can have a significant negative impact on the accuracy and quality of medical diagnosis. In a typical medical imaging system there are several phases, and in each of these phases unwanted noise can be introduced. The first phase is the actual modality or source that produces the medical image. Examples of such modalities include X-ray machines, computed tomography (CT) scanners, ultrasound scanners, magnetic resonance imaging (MRI) scanners, and positron emission tomography (PET) scanners. As for any sensor system or measurement device, there is always some amount of measurement noise present due to imperfections of the device or even due to physical limitations (such as statistical uncertainty). A lot of effort has been put into devices that produce low-noise images or image data. For example, images from digital detectors (very alike to CCDs in digital cameras) used for X-rays are post-processed to remove noise by means of flat field correction and dark field correction.
Once the medical image is available, this image is to be viewed by a radiologist. Traditionally light boxes were used in combination with film, but nowadays more and more display systems (first CRT-based and afterwards LCD-based) are used for this task. The introduction of those digital display systems not only improved the workflow efficiency a lot but also opened new possibilities to improve medical diagnosis. For example: with display systems it becomes possible for the radiologist to perform image processing operations such as zoom, contrast enhancement, and computer assistance (computer aided diagnosis or CAD). However, also significant disadvantages of medical display systems cannot be neglected.
Contrary to extremely low noise film, display systems suffer from significant noise. Matrix based or matrix addressed displays are composed of individual image forming elements, called pixels (Picture Elements), that can be driven (or addressed) individually by proper driving electronics. The driving signals can switch a pixel to a first state, the on-state (luminance emitted, transmitted or reflected), to a second state, the off-state (no luminance emitted, transmitted or reflected). For some displays, one stable intermediate state between the first and the second state is used—see EP 462 619 which describes an LCD. For still other displays, one or more intermediate states between the first and the second state (modulation of the amount of luminance emitted, transmitted or reflected) are used. A modification of these designs attempts to improve uniformity by using pixels made up of individually driven sub-pixel areas and to have most of the sub-pixels driven either in the on- or off-state—see EP 478 043 which also describes an LCD. One sub-pixel is driven to provide intermediate states. Due to the fact that this sub-pixel only provides modulation of the grey-scale values determined by selection of the binary driven sub-pixels the luminosity variation over the display is reduced.
A known image quality deficiency existing with these matrix based technologies is the unequal light-output response of the pixels that make up the matrix addressed display consisting of a multitude of such pixels. More specifically, identical electric drive signals to various pixels may lead to different light-output output of these pixels. Current state of the art displays have pixel arrays ranging from a few hundred to millions of pixels. The observed light-output differences between (even neighboring) pixels is as high as 30% (as obtained from the formula (minimum luminance−maximum luminance)/minimum luminance).
These differences in behavior are caused by various production processes involved in the manufacturing of the displays, and/or by the physical construction of these displays, each of them being different depending on the type of technology of the electronic display under consideration. As an example, for liquid crystal displays (LCDs), the application of rubbing for the alignment of the liquid crystal (LC) molecules, and the color filters used, are large contributors to the different luminance behavior of various pixels. The problem of lack of uniformity of OLED displays is discussed in US 20020047568. Such lack of uniformity may arise from differences in the thin film transistors used to switch the pixel elements.
EP 0755042 (U.S. Pat. No. 5,708,451) describes a method and device for providing uniform luminosity of a field emission display (FED). Non-uniformities of luminance characteristics in a FED are compensated pixel by pixel. This is done by storing a matrix of correction values, one value for each pixel. These correction values are determined by a previously measured emission efficiency of the corresponding pixels. These correction values are used for correcting the level of the signal that drives the corresponding pixel.
It is a disadvantage of the method described in EP 0755042 that a linear approach is applied, i.e. that a same correction value is applied to a drive signal of a given pixel, independent of whether a high or a low luminance has to be provided. However, pixel luminance for different drive signals of a pixel depends on physical features of the pixel, and those physical features may not be the same for high or low luminance levels. Therefore, pixel non-uniformity is different at high or low levels of luminance, and if corrected by applying to a pixel drive signal a same correction value independent of the drive value corresponds to a high or to a low luminance level, non-uniformities in the luminance are still observed.